Continuous calciner

ABSTRACT

A continuous calciner, particularly advantageous in converting gypsum to plaster, with two coaxial cylindrical walls, provides for heating air within the inner cylinder and then directing the air, for conveying and heating fine particles, along a spiral path, between the walls of the two coaxial cylinders, extending from one end of the calciner to an exit at the second end.

This invention relates to an apparatus for heating and conveying fineparticulate material in a chamber between the walls of two coaxialcylinders while heating the conveying air within the inner cylinder, andparticularly to the calcination of gypsum therein.

Calcination of gypsum, in its most common commercial form, involvesheating gypsum, calcium sulfate dihydrate, and driving off some of thewater of crystallization to form stucco, calcium sulfate hemihydrate. Onsubsequent addition of water to stucco, it will return to a rock-likecalcium sulfate dihydrate. Although this basically is a very well knownand ancient set of reactions, the stucco formed by calcination of gypsumcan vary in its characteristics in many significant ways, depending onthe method and apparatus employed. Several forms of heating devices havebeen used successfully to convert gypsum to stucco, such as large rotarykilns, kettles, and the multi-deck continuous calciner of Skinner et al,U.S. Pat. No. 2,788,960. Many other forms of heating devices have beentried resulting in producing very poor quality of useless calcinedgypsum. Successful calcination normally requires the uniform removal ofthree-fourths of the combined water of substantially all of the gypsumbeing calcined. The presence of excess overburned or underburnedmaterial is one potential problem in calcining gypsum. Rehydration ofthe stucco with moisture formed by the calcination is another problem.Condensation followed by agglomeration of the solids on any elements ofthe calciner that are not kept suitably heated presents another problem.

It is an object of this invention to provide a novel apparatus forheating fine particulate material with a high degree of uniformity andefficiency.

It is a further object to provide an apparatus for improved manufactureof plaster from gypsum.

It is a still further object of the invention to provide a novel methodof calcining gypsum.

These and other objects and advantages of the invention will be morereadily apparent when considered in relation to the preferred embodimentof the invention as set forth in the specification and shown in thedrawings in which:

FIG. 1 is a vertical cross section, partially diagrammatic, of theapparatus of the present invention.

FIG. 2 is a horizontal cross section of the calciner of FIG. 1, taken online 2--2.

Referring to the drawings, FIG. 1 includes a calciner 10, cycloneseparator 12 and a dust bag collector 14 for recovering fines. Theseparator 12 and collector 14 are of standard construction providing theusual function of separating the solids from the gases.

The novel calciner 10 includes an outer cylindrical wall 16 and an innercoaxial cylindrical wall 18 which form between them the calcination zone20. The calcination zone is divided into a plurality of spirallinglevels by spiral plates 22 which function to guide material in a spiralpath as it passes through the calcination zone 20.

Inside the lower portion of the inner cylindrical wall 18 is the heatsource 24 of calciner 10. The heat source 24 extends through thecalciner bottom wall 26 and consists of a cylindrical combustion chamber28 with a oil burner 30 affixed at the bottom.

Burner 30 is of a standard commercial construction. Burner 30 isequipped with a pilot light gas inlet 32, a combustion air inlet 34, afuel oil inlet 36 and an atomizing air inlet 38 for atomizing of thefuel oil. A gas burning pilot light is kept lit prior to starting theoil fire and throughout the burning of the oil by the burner 30, as iscommon with oil burners. The atomized fuel oil, thoroughly mixed withthe combustion air is caused to ignite and burns thoroughly in thecombustion chamber 28, in accordance with standard oil burner processes.Natural gas or other suitable fuels may be used for the primary fuel, ifpreferred.

Combustion chamber 28 has an upwardly pointing conical top deflectorwall 40 having a base diameter equal to the diameter of the cylindricalwall 42 of combustion chamber 28. An opening 44 is provided between thebase 46 of deflector wall 40 and the top edge 48 of cylinder wall 42,which opening 44 extends substantially around the full circumference ofthe combustion chamber 28.

Inside the upper portion 50 of the inner cylindrical wall 18 there isparticle conveying gas which has entered upper portion 50 through inlet52 in a top wall 54. As will be discussed later, this particle conveyinggas is a mixture of air, superheated steam and products of combustionbeing returned to the calciner after being separated from calcinedmaterial produced in a previous cycle.

Extending outward from the outer surfaces of combustion chamber wall 42are spiral plates 56, which function to guide the gas in upper portion50 in a spiral path as it passes from upper portion 50 to openings 58 inthe bottom of inner cylindrical wall 18. As the gas from upper portion50 moves downward, it is deflected radially outwardly by conical topdeflector wall 40 and it mixes with the hot products of combustionexiting from the combustion chamber 28 through opening 44. The gas fromupper portion 50 is also heated by the heat of the deflector wall 40 andthe spiral plates 56.

The spiral plates 56 are arranged with plates 56 at progressively lowerpositions being at an increasing spiral angle. The mixture of the gasand the products of combustion are caused to move in a spirallingdirection which is the same rotary direction as the upwardly spirallingcaused by spiral plates 22 in the calcination zone 20.

Uncalcined gypsum is continuously fed, in a finely ground form, throughfeeder 60 and inlet 62. Inlet 62 is in the bottom portion of the outercylindrical 16, and causes the uncalcined gypsum powder to be propelledby the moving hot gases and products of combustion that are moving fromthe openings 58 along the spiral paths upward through the calcinationzone 20.

As the uncalcined gypsum is carried spirally upward through thecalcination zone 20, it is also being very uniformly heated by the gasesand products of combustion which convey it. At the top of thecalcination zone 20, there are openings 64 in the outer cylinder 16through which the fine gypsum, gases and products of combustion exitfrom the calcination zone 20, and enter a duct 66.

Duct 66 conveys material from openings 64 to the separator 12. Separator12 is a cyclone collector which separates the solids, except for finedust, from the gaseous conveying medium. These duct laden gases are thenreturned by duct 68 through high volume blower 70 to inlet 52, fromwhere the cycle repeats itself. A portion of the returning gases aredirected to a bag dust collector 14, through pipe 71, where the finedust is removed and the clean gases and the water released from thegypsum are vented through outlet 72 to the atmosphere, to balance thevolume of gases added as products of combustion and as water ofhydration in the gypsum.

Although the calciner 10 may be used to calcine or dry other fine solidsalso, it has proven highly advantageous in calcining finely groundgypsum, which is calcium sulfate dihydrate (CaSO₄ ·2H.sub. 2 O). In theembodiment disclosed, a gypsum powder of about 90% through a U.S.Standard Sieve Size No. 100 is supplied continuously to a controllablyfixed speed feeder 60.

To produce two thousand pounds (907 kg) of calcined gypsum (calciumsulfate hemihydrate) per hour, uncalcined gypsum of 90% purity was fedto the inlet 62 at a uniform rate equal to about 2,340 pounds (1060 kg)per hour.

Recycled gases enter upper portion 50 at a rate of about 1500 cubic feetper minute (43 cu m/min) and at a temperature of about 340° F (170° C).These gases are deflected radially outward by deflector wall 40 as theymove downward. As these gases pass opening 44, they mix with theproducts of combustion which are exiting from the combustion chamber 28through openings 44. These products of combustion may be as much asabout 4000° F (about 2000° C).

The mixture of the two, recycled gases and gases which are the immediateproduct of combustion, moves downward between combustion chamber wall 42and inner cylinder and inner cylindrical wall 18. As this gas mixturemoves down, spiral plates 56 change the movement of the gases fromstraight downwardly to spirally downwardly, increasing the angle ofspiral gradually as it progresses downwardly. Spiral plates 56 alsoconduct heat from combustion chamber wall 42, to where the downwardlymoving gases are able to be further heated by the plates 56, as well asthe wall 42.

Looking downward, the downwardly moving gases are caused to move in aclockwise spiral. As these gases reach the bottom, they move radiallyoutward through openings 58 while continuing the clockwise rotarymotion. After passing through openings 58, the gases start a clockwisespiral motion upwardly through calcination zone 20, taking with them theuncalcined gypsum being fed thereinto at inlet 62.

As the gases pass through openings 58, the temperature of the gases isconstantly monitored. A preferred temperature of gases at openings 58,for the calcination of gypsum, is about 870° F (465° C). These 870° Fgases transfer a substantial portion of their heat to the 2340 pounds(1063 kg) of uncalcined gypsum being fed to the calcination zone, perhour.

Although not shown, it will be understood that all of the exteriorsurfaces of calciner 10, separator 12, bag collector 14, and ductstherebetween are covered with insulation to eliminate as much loss ofheat from the process as is reasonably possible.

To raise the heat of the 1500 cubic feet per minute incoming gases from340° F to 870° F there is required 650,000 BTU/hr. The combination ofthis 1500 cubic feet per minute of recycled gases with the products ofcombustion, at the 870° F converts the 2340 pounds of uncalcined gypsumof 90% purity to completely calcined calcium sulfate hemihydrate, CaSO₄·1/2H₂ O, with no measurable overburned or underburned material present,which is most unusual in the production of the hemihydrate.

The 870° F starting temperature for the calcination process iscontrolled by control of the rate of oil fed to the burner, 30. The 340°F final temperature of the gases, returning through inlet 52 aftercompleting a cycle, is controlled by varying the rate of feed ofuncalcined gypsum at inlet 62. If the returning gases increase intemperature, the product feed through inlet 62 is increased in order tobring the returning gas temperature back down to the desired reading.

The very compact arrangement of combustion chamber within thecalcination zone results in a most economical process.

Having completed a detailed disclosure of a preferred embodiment of myinvention so that those skilled in the art may practice the same, Icontemplate that variations may be made without departing from theessence of the invention.

I claim:
 1. Apparatus for uniformly heating a continuous supply of fineparticles comprising an elongate cylindrical outer wall, a substantiallycoaxial elongate cylindrical inner wall within said outer wall, meansfor directing a flow of material spirally through the elongate spacebetween said inner wall and said outer wall in a direction from a firstend of said apparatus to a second end of said apparatus, means forsupplying air to within the cylindrical inner wall and conveying saidair within said inner wall from said inlet to said first end, meanswithin said inner wall for heating said air within said inner wall,means at said first end for conducting said heated air from within saidinner wall to between said inner wall and said outer wall, and means atsaid first end for continuously feeding said fine particles to be heatedinto said heated air whereby said heated air conveys said fine particlesspirally through the length of said elongate space between said innerwall and said outer wall from said first end to said second end anduniformly heats all of said fine particles, means for removing saidheated fine particles from said apparatus, means for separating saidfine particles from conveying air and means for returning a substantialportion of said air to be reheated by said heating means.
 2. Theapparatus of claim 1 wherein said means for conducting said heated airfrom within said inner wall to between said inner wall and said outerwall is located near the bottom of said elongate space between saidinner and said outer wall whereby said fine particles are carried upwardtherethrough.
 3. The apparatus of claim 1 wherein said heating meansincludes a cylindrical combustion chamber wall spaced radially inwardfrom said elongate cylindrical inner wall.
 4. The apparatus of claim 3wherein a plurality of metal plates extend outwardly from saidcombustion chamber wall for heating gases passing adjacent thereto andcooling walls of said combustion chamber.
 5. The apparatus of claim 4wherein said metal plates are disposed at an angle to induce spiralmotion to said gases passing adjacent thereto.